There is widely known a method of isolating intake beds in deep wells (cf. e.g. Text "Isolation of Intake Beds in Deep Wells", by V. I. Krylov, Nedra Publishers, Moscow, 1980, pp. 258-259), wherein a cement slurry is pumped to a drill pipe string from a wellhead. Filling and thickening agents or cement setting accelerators are transported together with the cement slurry in polyethylene or rubber containers. Once the cement slurry has left the drill pipe string, the sheathing of the container is slit with the aid of knives, and additional components are thereafter introduced into the cement slurry to provide for flash setting and thickening.
The cement setting accelerators change rheological and structural-strength properties of the cement slurry not instantly but rather within a certain period of time, which causes the cement slurry flowing in the hole below drill pipes and in the lost-circulation formation to be inevitably mixed with drilling mud in the hole and with formation fluid in the lost-circulation formation since it enters the latter mostly through its well-drained channels. The cement slurry essentially losses its plugging properties, thus adversely affecting the quality of isolation of intake beds in drill holes (or wells).
There is further known a method of well cementing (cf. e.g. USSR Inventor's Certificate No. 1,055,856), wherein a well is first washed and thereafter a cement slurry is pumped into a drill pipe string from a wellhead, after which flash setting and thickening of the cement slurry is ensured with phases of higher and lower densities obtained by centrifugal separation of the cement slurry once it has left the drill pipe string. With a rotary motion imparted, the cement slurry enters the annular space. When flowing up to the wellhead, the rotating cement slurry flow gains an additional momentum. It has the highest rotation speed in the zone of minimum annular section between the well wall and the maximum outer diameter of the separator. This results in excess pressure being exerted on the well walls. As a result, the phase of higher density is again involved in intensive mixing with that of lower density, thus affecting isolation of intake beds. The most intensive mixing of the phases of higher and lower densities occurs when isolating highly permeable intake beds.
There is also known a device for well cementing (SU, A No. 1,055,856), comprising a housing connected with a lower end of the drill pipe string by means of an adapted and having an axial passage and at least a single lateral circulation duct for separating the cement slurry. The circulation duct is provided on the outside surface of the housing in a spiral and connects the axial passage with an annular space, wherein the cement slurry undergoes centrifugal separation. The housing is likewise provided with a case, whose outside surface above the lateral circulation duct is a paraboloid of revolution with a cross-section increasing upwards.
For effective rotation of the cement slurry flow it is necessary to provide a considerable pressure drop across the spiral circulation ducts, for which the ducts must be long enough and have a small cross-section. This is fairly difficult to achieve and involves hazard in isolating intake beds. In addition, more stringent demands are consequently to be placed on cementing equipment (pressure piping of a drillsite, drill pipe string).
The cross-section of the cement slurry flow noticeably decreases in the annular channel, gradually narrowing upwards, between the well walls and the casing made as a paraboloid of revolution, which gives rise to high turbulization of the flow hampering centrifugal separation of the cement slurry. In addition, a smaller cross-section of the upward flow of the cement slurry in the annular space leads to higher pressure exerted on a lost-circulation formation, which restricts the possibility of ensuring higher rotation speed of the cement slurry, in particular, when isolating highly permeable intake beds and promotes downward movement of the lower density phase followed by penetration of this phase into the intake bed and mixing with the phase of higher density.
In the case of an embodiment of the device with a single circulation duct, no dynamic balancing is carried out, which fails to provide a stable concentric position of the device in a drill well.